1. Field of the Invention
The present invention relates to an optical duo-binary transmission apparatus using an optical duo-binary transmission method, and more particularly to a precoder performing parallel processing and an optical duo-binary transmission apparatus using the same.
2. Description of the Related Art
A Dense Wavelength Division Multiplexing (hereinafter “DWDM”) optical transmission system has excellent communication efficiency, since it can transmit an optical signal having multiple channels with different wavelengths through a single optical fiber. Transmission speed is not a limiting factor, because an optical signal travels at the speed of light through the optical medium. Accordingly, DWDM systems are now widely used in ultra-high speed internet networks, and data traffic on such networks is increasing. Systems in common use employing DWDM technology, as of late, are each capable of transmitting more than a hundred channels through a single optical fiber. Furthermore, various research efforts are being actively conducted to develop a system which can transmit more than two hundred 40-gigabits-per-second (Gbps) channels through a single optical fiber simultaneously, for an overall transmission speed on the order of 10 terabits-per-second (Tbps).
In meeting a rapid increase of data traffic and a request for high-speed transmission of data of more than 40 Gbps, however, the enlargement of transmission capacity is restricted due to severe interference and distortion between channels if the channel distance is less than 50 GHz when a light intensity is modulated using the conventional non-return-to-zero (NRZ) method. In particular, transmission distance is restricted in high-speed transmission of more than 10 Gbps since a direct current (DC) frequency component of a conventional binary NRZ transmission signal and a high frequency component spread during modulation causes non-linearity and dispersion when the binary NRZ transmission signal propagates in an optical fiber medium.
Recently, an optical duo-binary technology has been highlighted as an optical transmission technology capable of overcoming restriction of transmission distance due to chromatic dispersion. A main advantage of the duo-binary transmission is that the transmission spectrum is reduced in comparison to the general binary transmission. In a dispersion restriction system, transmission distance varies inversely with the square of the transmission spectrum bandwidth. When the transmission spectrum is halved, for example, the transmission distance quadruples. Furthermore, since a carrier frequency is suppressed in a duo-binary transmission spectrum, it is possible to relax the restriction of an optical power output caused by Brillouin scattering excited in the optical fiber.
FIG. 1 is a block diagram showing one construction of a conventional optical duo-binary transmission apparatus. Hereinafter, the conventional optical duo-binary transmission apparatus will be described with reference to FIG. 1.
In FIG. 1, the conventional optical duo-binary transmission apparatus includes a multiplexer 10, a precoder 20, a low pass filter 30, a modulator driving amplifier 40, a laser source 50 for outputting a carrier wave, and a Mach-Zehnder interferometer type optical intensity modulator 60. The multiplexer 10 multiplexes data input signals of N number of channels so as to output the multiplexed signal, and the precoder 20 codes the multiplexed signal. The low-pass filter 30 converts a 2-level binary signal outputted from the precoder 20 into a 3-level electrical signal, and reduces the bandwidth of the signal. The modulator driving amplifier 40 amplifies the 3-level electrical signal to output an optical modulator driving signal.
The input signals of N number of channels are multiplexed by the multiplexer 10, and the multiplexed signal is then coded by the precoder 20. The 2-level binary signal outputted from the precoder 20 is inputted to the low-pass filter 30, the low-pass filter having a bandwidth corresponding to about ¼ of a clock frequency of the 2-level binary signal. This excessive limitation to the bandwidth causes interference between codes, which thus changes the 2-level binary signal to a 3-level duo-binary signal. The 3-level duo-binary signal is amplified by the modulator driving amplifier 40 so as to be used as a driving signal of the Mach-Zehnder interferometer type optical intensity modulator 60. The carrier wave outputted from the laser source 50 is subjected to phase and optical intensity modulation according to the driving signal of the Mach-Zehnder interferometer type optical intensity modulator 60 and is then outputted as a 2-level optical duo-binary signal.
FIG. 2 shows a position of the precoder 20 in the optical duo-binary transmission apparatus. Referring to FIG. 2, the N number of input signals are time-multiplexed by the multiplexer 10, and the multiplexed signal is coded by the precoder 20. Accordingly, transmission speed increases N times in comparison with transmission speed before the multiplexing. This means that a high-speed precoder is necessary.
FIG. 3 is a view showing a structure of the conventional precoder 20. The precoder 20 includes one exclusive OR (XOR) gate 21 and one data bit delayer 22. When a signal having a data sequence of 11010111101010 is inputted to the precoder 20, the precoder outputs a signal having a data sequence of 1001101011100111 as shown in FIG. 4. That is, the precoder 20 toggles the previous output signal whenever the input signal becomes 1.
However, according to the prior art, in the case of a high-speed data signal, it is difficult to realize a high-speed precoder due to time delay and limitation in speed of the XOR gate constituting the precoder.